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B. Lendl, M. Brandstetter:
"New approaches and applications for the analysis of liquids using mid-IR quantum cascade lasers";
Hauptvortrag: Miomd Xi, Chicago (eingeladen); 04.09.2012 - 09.09.2012.

Mid-IR spectroscopy allows direct determination of analytes on basis of their highly characteristic infrared spectrum. Due to the narrow ro-vibrational absorption lines present in small gaseous molecules, single wavelength light source, like distributed feedback QCLs, often provide sufficient selectivity for highly specific analyte determination even in a complex matrix. When analytes in liquids have to measured the situation is different for several reasons. First of all the solvent in which the analyte is present will also contribute to the overall attenuation observed when probing the sample. Secondly, absorption bands in liquids are usually several tens of wavenumbers broad and as a consequence it is most likely that absorption features of several molecules present in the sample will overlap. To account for these particularities either sample pretreatment steps before analysis are needed or measurements at several wavelengths are required. Mid-IR spectroscopic measurements can either be carried out in transmission or using different variants of evanescent wave spectroscopy. A significant advantage of using quantum cascade lasers as compared to conventional thermal mid-IR light sources is their high spectral power density, which allows for increased path lengths in liquid phase analysis. As a consequence more sensitive and more robust analyzers for liquids can be designed.
A good example is the QCL based analyzer "ERACHECK" for measuring total petroleum hydrocarbons (TPH) in water. This analyzer probes the C-H deformation bands of oil extracted from waste water samples into the solvent cyclohexane. Based on this new method an ASTM test method (ASTM 7678-11) for TPH has recently been established, being the first official standard requiring the use of QCLs. Recently, this method has been extended for quantification of oil in soil, where again extraction of the sample with cyclohexane is required prior to measurement. Both applications benefit from the fact that a clean-up step is part of the analysis sequence. This step transfers the analyte into a clean solvent and removes matrix components. Therefore, measurement in a narrow spectral range is sufficient for successful quantification of the target analyte.
For direct determination of single or multiple analytes in liquids measurements have to be carried out at several wavelengths in order to assure selectivity of analysis. Here data analysis requires multivariate (chemometric) methods such as partial least square calibration (PLS), which derive the desired analyte concentration from the recorded absorption values. During chemometric model development and model validation an additional analytical reference method is required that provides independent information on the analyte concentrations of the investigated samples. Once a chemometric model has been established, direct and thus rapid analysis is possible only on basis of the spectroscopic data. A portable, fully automated set-up requiring only thermoelectric cooling of the used laser and detector has been developed for point-of-care analysis of human sera.